ESEEM spectra and their analysis by computer simulation were obtained for the deoxyCo-substituted subunits of four Fe-Co hybrid hemoglobins (Hbs): deoxy(Co-Fe)2 and (FeCo-deoxyCo)2. From comparison with model compounds, ESEEM components arising from the proximal imidazole Nk were assigned. These were found to differ for the Co subunits of an unligated Hb as compared to a di-ligated Hb, while the ESEEM of the proximal Nk in the ~Co subunits was invariant with the ligation state of the Hb. All deoxyCo subunits show similar 14Nk nuclear quadrupole coupling parameters while the nuclear hyperfine coupling constant for the subunits in an unligated Hb is reduced by w 0.15 MHz as compared to the others.